Optical transmission systems are generally based on direct modulation or external modulation of an optical input signal. External modulation is preferred for high speed applications.
External modulation of an optical input signal involves applying an electrical modulating signal to a continuous wave (CW) optical signal provided by an optical source, such as a laser. Electro-optic modulators, such as Mach-Zehnder interferometers (MZIs), are typically used for high speed applications.
A Mach-Zehnder modulator controls the amplitude of an optical signal. An input waveguide is split into two waveguide interferometer arms. When a voltage is applied across one of the waveguide arms, a phase shift is induced for the optical signal in that waveguide arm. The optical signals from the two waveguide interferometer arms are then recombined. Changing the electric field on the phase modulating path determines whether the two optical signals interfere constructively or destructively at the output, thereby controlling the amplitude or intensity of the optical output signal.
A ring-assisted Mach-Zehnder interferometer (RAMZI) has the same behavior as a Mach-Zehnder interferometer, but uses a ring to modify the phase. The ring may be active or passive to modify the phase. The ring-assisted Mach-Zehnder interferometer has the same power in each waveguide arm, and the phase in each waveguide arm is changed using modulation diodes. A phase difference results in a modulation of amplitude. The ring in the Mach-Zehnder interferometer is used to improve the linearity of the modulator, which in turn improves the extinction ratio. A change in the power amplitude is seen as a side effect, which is to be reduced in the ring-assisted Mach-Zehnder interferometer.
As an alternative to the Mach-Zehnder modulator and the ring-assisted Mach-Zehnder interferometer, a segmented optical modulator is disclosed in U.S. Pat. No. 7,515,778. The optical modulator includes an adjustable drive arrangement for dynamically adjusting the effective length of the optical signals paths within the modulator. Each modulator arm is partitioned into a plurality of segments, with each segment coupled to a separate electrical signal driver. The effective length of each modulator arm will be a function of the number of drivers that are activated for each arm at any given point in time. A feedback arrangement may be used with the plurality of drivers to dynamically adjust the operation of the modulator by measuring the extinction ratio as a function of optical power, and turning on or off individual drivers accordingly.
The above approaches for modulating an optical input signal are based on controlling the phase differences between the optical waveguide arms which creates the difference in amplitude in the optical output signal. The optical waveguide arms typically need to be a sufficient length in order to have a phase difference that produces a ratio P(bit1)/P(bit0) at the output of the Mach-Zehnder modulator large enough to meet the specification of the link. This ratio is referred to as the Extinction Ratio. For typical PN modulation diodes used in silicon photonics, the minimum length is typically 0.3 mm at a voltage of 1.8V. This effects the size and compactness of electro-optic (E/O) devices. Even though more advanced devices may be used to create a better difference of phase, they are often more complicated and expensive to make.